In situ thermal, optical, and structural investigations on reversible thermochromism in bismuth molybdate

Reversible thermochromic inorganic materials show stable and perceptible color change with changing environmental temperatures. This property makes them excellent materials for fabricating temperature indicators, military camouflage materials, and thermal warning devices. However, the mechanism of thermochromism has not been fully understood. Here, we prepared novel bismuth molybdate materials by calcination of Bi2O3 and MoO3 mixtures. The materials exhibit reversible thermochromism, gradually changing color from milky white at 25 degrees C to bright yellow at 500 degrees C. The phase composition of the product is tunable by adjusting the ratio of Bi2O3 and MoO3 in the starting materials, producing either single phase of alpha-Bi2Mo3O12 or mixed phases of alpha-Bi2Mo3O12, beta-Bi2Mo2O9 and gamma-Bi2MoO6. The mechanism of reversible thermochromism was studied by in situ UV/Vis diffuse reflectance spectroscopy (UV/Vis DRS), in situ synchrotron radiation powder X-ray diffraction (SRPXRD), and differential scanning calorimetry (DSC). In situ SRPXRD revealed no phase transition during heating and cooling, which agrees with DSC analysis with no thermal event detected until the melting temperature of alpha-Bi2Mo3O12 at 661.6 degrees C. With increasing temperature, in situ SRPXRD also revealed anisotropic expansion of the alpha-Bi2Mo3O12 lattice parameters, while in situ UV/Vis DRS showed a continuous red shift in the absorption edge and gradual decrease of band gap, suggesting that lattice expansion and shrinking upon heating and cooling is the main reason for the thermochromic behavior in bismuth molybdate materials.

Automated summary

Novel bismuth molybdate materials exhibit reversible thermochromism, with color changing from milky white to bright yellow as the temperature increases. The phase composition of the product can be adjusted by changing the ratio of Bi2O3 and MoO3 in the starting materials, leading to either single phase or mixed phases. The mechanism of reversible thermochromism in bismuth molybdate materials is mainly attributed to lattice expansion and shrinking upon heating and cooling.